This invention relates to processes for regeneration of acid halide solutions for leaching and digestion of ores for recovery of mineral products, or for use in cleaning metallic products such as steel sheets. In particular, the invention relates to a process for regenerating an aqueous hydrohalic acid solution from pyrohydrolysis of an aqueous metal halide solution in an oxygen-enriched atmosphere.
The use of hydrohalic acids, primarily hydrochloric acid and hydrofluoric acid, for leaching and digestion of ores for mineral recovery has recently become more prevalent. This may partially be due to the development of acid resistant plastic which has made it feasible to transport hydrohalic acids, and due to the fact that they are better suited to the leaching and digestion of some types of ores than other acids such as sulfuric acid.
Leaching processes utilizing hydrohalic acids generate large amounts of spent aqueous metal halide leach liquors which must be dealt with. Processes for recovery of acid halides from spent leach liquors have been developed in which the leach liquor is hydrolyzed at an elevated temperature (i.e. xe2x80x9cpyrohydrolyzedxe2x80x9d) in an air roaster. The pyrohydrolysis process involves passing the spent aqueous halide leach liquors into the roaster to generate a solid metal oxide and an off-gas containing hydrogen halide and water vapor. The hydrogen halide produced by the pyrohydrolysis is recovered by absorption in water to form a regenerated hydrohalic acid solution which can then be used to leach or digest more ore.
The pyrohydrolysis process can be represented by the following formulae where the halide is chloride:
CnHm+(n+m/4)O2xe2x86x92nCO2+(m/2)H2O
MeClx+(x/2)H2Oxe2x86x92MeOx/2+xHCl
H2O(l)xe2x86x92H2O(g)
In order to provide the required energy to evaporate the spent aqueous halide leach liquor and provide an elevated temperature for the pyrohydrolysis to occur, existing roasters burn a hydrocarbon fuel in air. In addition to hydrogen halide gas and water generated by pyrohydrolysis of the leach liquor, the off-gas also contains gases produced by combustion of the hydrocarbon fuel, including carbon dioxide and nitrogen. In fact, the off-gas can contain 50 to 70% nitrogen, depending on the type of fuel and the concentration of the feed halide solution.
Present day air roasting processes have made gains in energy efficiency by recovering much of the sensible heat from the roaster off-gas, such that the portion of the off-gas which is discharged to the atmosphere is cooled to about 100xc2x0 C. This is accomplished by contacting the off-gas with fresh halide leach liquor, resulting in heating of the halide leach liquor and cooling of the off-gas. In addition, flow of the hot off-gas over the fresh halide solution results in partial evaporation and concentration of the halide solution. Dilution water (at ambient temperature) is added to the halide solution prior to pyrohydrolysis to compensate for evaporation and to prevent crystallization of metal salts which would cause blockages in the process equipment.
Thus, while presently used air roasting processes are relatively energy efficient due to heat recovery from the off-gas, it would be desirable to make the roasting process even more energy efficient.
The present invention overcomes at least some of the problems of the prior art discussed above by providing a process for forming an aqueous hydrohalic acid solution by pyrohydrolysis of a spent aqueous metal halide leach liquor in an air roaster, in which the energy requirements for pyrohydrolysis are provided by combustion of a hydrocarbon fuel with an oxygen-enriched gas. The terms xe2x80x9coxygen-enriched gasxe2x80x9d and xe2x80x9coxygen-enriched oxidizing gasxe2x80x9d as used herein refer to a gas for combustion in which the concentration of oxygen is greater than the concentration of oxygen in air, which is about 21 percent by volume.
A given volume of an oxygen-enriched gas is smaller in total volume than a given volume of air having the same oxygen content. Thus, the use of an oxygen-enriched gas in place of air reduces the volume of gas required to burn the hydrocarbon fuel in the roaster, and also results in a corresponding decrease in the amount of combustion off-gas produced by combustion of the fuel. The reduced volume of the off-gas results in a number of benefits. For example, the smaller gas volumes permits the use of smaller, less costly process equipment. Also, the off-gas contains a higher concentration of hydrogen halide gas, resulting in a more concentrated hydrohalic acid solution after absorption of the gas in water. Further, since some heat energy is lost in the off-gas released to the atmosphere, reducing the off-gas volume also improves the energy efficiency of the process.
The inventors have also identified some additional benefits of reducing the off-gas volume which are not readily apparent. These additional benefits relate to the effect of the off-gas volume on the volume of the incoming feed solution that can be processed. As mentioned above, recovery of heat energy from the roaster off-gas is accomplished by contacting the off-gas with fresh halide leach liquor, resulting in partial evaporation and concentration of the halide solution. The evaporated water is entrained in the off-gas and is subsequently released to the atmosphere, taking with it some heat energy. Dilution water is added to the halide solution to replace the water evaporated from the halide solution and to prevent crystallization of the metal salts in the solution. Thus, the latent heat required to evaporate the dilution water, which is considerable, represents energy which is wasted.
Furthermore, some of the oxide produced in the roaster is entrained as dust in the off-gas. The oxide dust is absorbed in the venturi by the fresh halide solution, thus increasing the concentration of the metal in the solution, and increasing the need for dilution water to prevent crystallization.
Having identified these problems associated with air roasting, the inventors have found that reducing the off-gas volume reduces the amount of evaporation of the fresh halide feed solution and also reduces the amount of entrained oxide dust in the off-gas, thereby significantly reducing the amount of dilution water which must be added to the halide solution to prevent crystallization. The reduced need for dilution water represents a significant gain in energy efficiency, making the use of oxygen-enriched gas more energy efficient and less costly than a corresponding air roasting process. The lack of appreciation of these benefits by others in this field may account for the fact that oxygen-enriched gas has not previously been used for roasting of metal halide solutions.
Another reason why oxygen-enriched roasting of halides has not been attempted is that it may have been believed that increasing the amount of oxygen in the oxidizing gas could result in excessive oxidation of the metal halides, leading to undesirable side products such as chlorine gas. The inventors have not detected increased amounts of such side products in the process of the invention.
In one aspect, the present invention provides a process for forming an aqueous acid halide-containing leachant from a spent aqueous metal halide leach liquor, comprising: (a) introducing into a roaster a hydrocarbon fuel, an oxygen-enriched oxidizing gas and said aqueous metal halide leach liquor; (b) pyrohydrolyzing the aqueous metal halide leach liquor in said roaster by combustion of said hydrocarbon fuel with said oxygen-enriched oxidizing gas to produce an acid halide-containing gas fraction and a metal oxide-containing solid fraction; (c) separating the gas fraction from the solid fraction; (d) recovering heat energy from the gas fraction; and (e) absorbing the gas fraction in water to produce said aqueous acid halide leachant.